We had a successful mission of the IRTS (InfraRed Telescope in Space), which was launched by the HII-3 rocket on March 18,1995. The IRTS observed about 7 resolution spectra of the diffuse Galactic light by the Mid-InfraRed Spectrometer (MIRS) aboard the IRTS.These provide the mid-IR spectra of the zodiacal light and the diffuse Galactic Light for the first time. We prepared several pieces of software prior to the launch in order to make the observation plan and confirm the flight performance of the MIRS.Based on the post-flight data analysis, we have clearly shown that the mid-infrared diffuse light around the Galactic plane consists of the so-called infrared unidentified bands (UIRs) of the 6.2,7.7,8.6, and 11.3 mm. This suggests the ubiquitous existence of the UIR emitting material in the interstellar space, which is thought to be carbonaceous material and provides key information for the investigation of the interstellar dust grain models. Together with the MIRS data reduction, we have analyzed the COBE data, finding that the mid-IR diffuse emission is proportional to the strength of the interstellar radiation field, which supports the temperature-fluctuating very small dust grain model for this emission. In parallel to this research, we have also shown that the polarization of the thermal emission could provide the information to separate the diffuse emission into several components. In summary, we have clearly established the new observational method for the research of interstellar dust grains by using the diffuse background emission and achieved the primary goal of this project. We are planning to make further analysis of the MIRS data and pursue the investigation of interstellar dust grains.